I. Field of the Invention
The present invention relates to a method and apparatus by which the efficiency of a multi-pump pumping system may be accurately determined, and the individual pumps thereof selectively energized and deenergized to optimize the efficiency of the system.
II. Description of the Prior Art
A typical pumping system may comprise a plurality of valves, fittings and pumps coupled between an inlet header of the system and an outlet header thereof. The pumps may be in parallel such that each pump suction port communicates directly, or through one or more valves, with the inlet header, and each pump discharge port communicates directly, or through one or more valves, with the outlet header. Alternatively, the pumps may be in series such that the suction port of a first pump is coupled to the inlet header, the discharge port of the first pump is coupled to the suction port of a succeeding pump, the discharge port of the last pump is coupled to the outlet header, and valves are supplied to control communication between the pumps and between the system headers. In both systems, the valves, pumps and headers would typically be coupled together with fittings or the like.
With either type of system, whether parallel or series, it has been known to selectively energize and deenergize selected ones of the pumps and to open or close related valves. By way of example, such selective control has been known for maintaining a predetermined range of flow through the system, a predetermined range of pressure at a delivery point downstream of the outlet header, or a sump level at the input header. Additionally, where the pumping system utilizes variable speed pumps, it has been known to control the speed of the energized pumps so as to maintain a system parameter constant such as at a delivery point. Such a system parameter might be pressure, flow, temperature, or elevation or the like, but is not necessarily limited thereto.
In effecting the selective energization and deenergization of the pumps, it might occur that energizing an additional pump in order to maintain flow, level or pressure or the like results in inefficient operation of the pumps and, thus, costly energy waste. Indeed, it is known that one new pump may be more efficient than two new pumps at certain rates of flow through the pumping system, whereas as the system ages two may become more efficient than one. Other factors may also affect individual pump performance and, hence, efficiency. As a consequence, it has been proposed to monitor pumping systems to continuously determine the so-called wire-to-water efficiency of the pumping system whereby over time the performance of the pumps may be evaluated and appropriate selection and/or replacement of pumps made in an effort to obtain the best possible efficiency for the system.
The wire-to-water efficiency determination for a single pump system has been previously proposed according to the formula: ##EQU2## wherein W is wire-to-water efficiency;
Q is flow rate through the system; PA1 H is pump head or differential pressure across the pump, i.e., between pump suction and pump discharge; PA1 OP is the overpressure downstream of the outlet header (typically=0 or ignored for variable speed pumps); and PA1 E is energy input to the system to drive the pump(s).
As explained in U.S. Pat. No. 4,120,033, the disclosure of which is incorporated herein by reference, the above described wire-to-water efficiency could be utilized to design, modify and/or control a pumping system.
Of importance is that the efficiency is determined by looking at the pump head, i.e. directly across the suction and discharge ports of the pump itself. Even in a multi-pump system, efficiency has traditionally been measured by looking at the individual pump heads or pump differential pressures between each of the suction and discharge ports thereof as shown, for example, in U.S. Pat. No. 4,584,654.
Such an approach requires numerous (and costly) transducers and communication equipment and/or may lead to erroneous efficiency determinations. For example, in order to evaluate pump head H for the purpose of determining efficiency, particularly in a parallel pump system, the pump head for only one of the pumps should be utilized. Thus, either one pump could be arbitrarily monitored or all the pumps could be monitored and the largest pump head, for example, selected. In any event, the selected pump head would be assumed to be the same for the other pumps. Not only does such an approach increase cost where all pumps are monitored but also increases complexity because of the circuitry required to determine which pump head to utilize. Moreover, utilizing only one pump head could lead to substantial inaccuracies in determining wire-to-water efficiency.
An additional problem with the foregoing efficiency determination is that it could lead to erroneous system design criteria as well. The foregoing typically did not account for certain system frictional losses such as losses due to valves and/or fittings. Rather, such losses were assumed to be constant at a given flow and/or pump head for the system and the designer might therefore include a fixed "fudge factor" into the pump system design criteria to account for such unknown losses Not only might the fudge factor be in error, for any given condition, it may not be possible to develop a fudge factor which is satisfactory for all conditions. Thus, errors could abound.